Upgrade Waitlist on Flight w/Several Segments

My wife and I will be flying IAH-ORD-FRA-FLR in a couple months. We are currenlty booked in E+. We would like to upgrade to business class solely to have a good seat on the overnight flight from ORD-FRA, but I understand it's not possible to upgrade a single segment of a trip that's booked altogether.

That being said, we don't want to spend the money to upgrade the entire trip. We are, however, willing to consider spending the $550 to upgrade with miles. Currently, cash+miles upgrades are available only on IAH-ORD; ORD-FRA is waitlist only. Assume we pay the $550 and 20,000 miles to upgrade/join the waitlist. Because we're traveling together and because this is a multi-segment flight, I have the following concerns:

1. Suppose my wife and I both get upgraded only on IAH-ORD but not on ORD-FRA. Will we forefeit our entire $550 + 20,000 miles per person?

2. Suppose, at the gate, only I climb the waitlist and get upgraded. May I decline the upgrade and be reimbursed the $550 + 20,000 miles? Also, if I decline the upgrade, will my wife and I still have the same E+ seats we selected at booking?

I know these are pretty fact-specific questions. I appreaciate any advice.